1. Field of the Invention
The present invention relates to a power supply apparatus and particularly to a power supply apparatus having the function of detecting an abnormality in electric current flowing in a drive circuit mounted on the power supply apparatus.
2. Description of the Background Art
Hybrid vehicles and electric vehicles have recently been of great interest as environment-friendly motor vehicles. A hybrid vehicle has, as its motive power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to secure the motive power source and a DC voltage from the DC power supply is converted by the inverter into an AC (alternating current) voltage to be used for rotating the motor and thereby securing the motive power source as well.
An electric vehicle refers to a motor vehicle that has, as its motive power sources, a DC power supply, an inverter and a motor driven by the inverter.
The hybrid vehicles and electric vehicles generally use a high-voltage power supply for producing high power. When the high-voltage power supply is used, overload could cause overheating to incur the danger that an electric motor seizes up or burns out. Moreover, when an electric leakage occurs, there arises the danger of an electrical shock. A safety device is thus required for avoiding these dangers (for example, see Japanese Patent Laying-Open No. 07-123504).
FIG. 11 is a block diagram showing a configuration of a safety device for electric vehicles (hereinafter referred to as EV safety device) disclosed in Japanese Patent Laying-Open No. 07-123504.
Referring to FIG. 11, the EV safety device is configured to have a switch 150 on a power feeding path L extending from a DC power supply 110 to a load circuit 130 and open/close switch 150 according to an external signal that is input from a protection circuit 140 to a drive circuit 151.
More specifically, in protection circuit 140, a current detector 141 detects electric current passing through power feeding path L. The output of current detector 141 is amplified by a current detection circuit 142 and input to a control circuit 143. At the time when predetermined operating time has passed since the time when the value of the detected electric current exceeds the rated electric current of load circuit 130, if the value of the detected electric current decreases to become equal to or lower than the rated current, control circuit 143 drives an output relay circuit 144 and turns off a contact r of switch 150 through drive circuit 151.
Here, the operating time refers to a time limit from the time when the value of the electric current detected by current detector 141 exceeds the rated electric current. The operating time is set to allow switch 150 to be opened if the detected current value does not fall to or below the rated current at the time when the operating time has passed. Further, the operating time is set, according to the magnitude of the passing electric current, so that the operating time is shorter in an inversely proportional manner as the current value is larger for example. If the detected electric current falls to or below the rated current within the time limit of the operating time, switch 150 is not opened. Then, the next time the detected current exceeds the rated current, the time limit is newly set.
Regarding the EV safety device shown in FIG. 11, if it is detected that the passing current exceeds the rated current and the passing current does not fall to or below the rated current within a predetermined operating time from the time of the detection, the power feeding to load circuit 130 is stopped to accordingly afford protection against overcurrent.
Until the time when the operating time has passed, the power feeding to load circuit 130 is continued. Thus, even if load circuit 130 is temporarily in an overloaded state, the power feeding to load circuit 130 is not immediately stopped. Therefore, such an inconvenience that protection circuit 140 operates in a normal state to stop the power feeding is avoided.
According to the method of detecting an abnormality illustrated in FIG. 11, however, whether the passing current is abnormal or not is determined based on the rated current and the operating time that is uniquely determined according to the magnitude of the passing current, and accordingly a problem arises in precision of detecting the abnormality in the following respects.
Specifically, as load circuit 130 in FIG. 11, if an inverter and an AC motor are provided, the passing current has a sinusoidal current waveform in a normal operation. When an abnormality occurs in control of the inverter, the passing current has its waveform considerably different from the one the current should have.
Examples of the passing current in an abnormal state include electric current having a current waveform temporarily exceeding the rated current to a considerably great degree and electric current having a current waveform continuing around the uppermost level of the sinusoidal wave. When large electric current that exceeds the rated current flows in the inverter, a large load is exerted temporarily on the inverter depending on the magnitude of the passing current and the period of time during which the current flows, which could break the inverter. In the case where the passing current continuously flows having its level around the uppermost level of the sinusoidal wave, the load with the maximum level in a normal state is continuously exerted on the inverter, which could also break the inverter. In order to prevent the inverter from being broken, it is necessary to surely determine that any current waveform that never occurs in a normal operating state is abnormal.
According to the aforementioned method of detecting an abnormality, any abnormal electric current that flows with its level temporarily exceeding the rated current to a considerably great degree is regarded as abnormal if such abnormal current flows for more than a predetermined operating time.
For such abnormal current flowing continuously with its level around the uppermost level of the sinusoidal wave, it is necessary to lower the threshold used as a reference in determining whether an abnormality occurs or not, from the rated current to the uppermost level of the sinusoidal wave. However, if the threshold value is set at the uppermost level of the sinusoidal wave, it is difficult to accurately detect the abnormal current flowing at and around the uppermost level of the sinusoidal wave, since the time-limit setting is initialized when the passing current falls to or below the threshold value within the time limit of the operating time and the time limit is newly set the next time the current exceeds the threshold value. In other words, depending on the setting of the operating time, if the operating time is relatively short, the passing current that temporarily exceeds the uppermost level of the sinusoidal wave that should merely exert a small load on the inverter could be detected as abnormal current. On the contrary, if the operating time is relatively long, any passing current that continuously flows with its level around the uppermost level of the sinusoidal wave and that exerts a large load on the inverter could not be detected as abnormal current, since the time-limit setting is initialized. As such, the above-described method of detecting an abnormality has a problem that the pattern of abnormal current that is not preferable for the inverter does not necessarily match the result of determining whether an abnormality occurs or not.